1. Field of the Invention
The invention relates generally to the field of electromagnetic induction well logging. More particularly, the invention relates to method and structure for generating and rapidly reversing a large magnetic dipole moment. The reversible magnetic dipole is used to generate transient induction signal in Earth's formations in order to study electromagnetic transient response related to the electrical properties of the Earth's formations surrounding the wellbore.
2. Background Art
Electromagnetic induction well logging is known in the art for determining electrical properties of earth formations penetrated by a wellbore. An instrument for electromagnetic induction logging is lowered into the well bore and typically includes a transmitter induction antenna connected to a source of current. The current source driving the antenna could be a sinusoidal current. In this case the alternating current (AC) measurements in frequency domain are performed. Alternatively the current source could be a generator of rectangular pulses with sharp trailing edge thus facilitating so called transient electromagnetic measurements in time domain. The induction instrument also includes at least one induction receiver antenna axially spaced apart from the transmitter antenna. An induction antenna, used for both transmitting and receiving signals in an induction well logging instrument can be thought of as a magnetic dipole. In a simplest case of a current loop or coil the direction of the magnetic dipole moment is perpendicular to the plane of the turns of wire making the coil or the loop.
An electromagnetic induction well logging instrument can be implemented as a “wireline” instrument or as a part of a drilling tool assembly: a measurement while drilling” (MWD) instrument. A wireline instrument is lowered into the wellbore and removed from there by means of an armored electrical cable. The parts of the instrument housing that cover the transmitter and the receiver antennas are made from electrically non-conductive and non-magnetic material so that these parts do not interfere with the magnetic field generated by transmitter antenna and do not mask the response from the earth formations coming to the receiver antenna. An MWD logging instrument has high strength metal collar that to a substantial degree affects the signal generated by transmitter antenna and the received signals. See, for example, U.S. Pat. No. 5,138,263 issued to Towle et al. A conductive part, for example, high strength metal rod could be also a part of a wireline instrument to facilitate using an additional logging instrument coupled bellow the induction logging instrument.
In order to increase depth of investigation of an induction logging instrument it is required for the transmitter antenna to generate strongest magnetic dipole moment possible. For this purpose the induction sensors often include a magnetically permeable material, such as ferrite. See, for example, previously cited U.S. Pat. No. 5,138,263 issued to Towle et al., as well as U.S. Pat. No. 6,452,388 issued to Reiderman, et al. and U.S. Pat. No. 5,757,186 issued to Taicher, et al. The effect of magnetically permeable material is the increase the dipole magnetic moment of the antenna per unit current in the antenna coil due to magnetization of the magnetic material. Yet another effect of a high permeability magnetic material is to concentrate magnetic flux, thus substantially preventing exposing conductive parts of the antenna assembly to alternating magnetic field of the antenna coil, which would otherwise produce undesired eddy currents in the conductive parts.
Increasing magnetic dipole moment per unit current by using high permeability magnetic material or by increasing the number of turns in the antenna coil typically results in higher inductance of the antenna. As a consequence, the voltage developed across the antenna terminals could be inappropriately high for down-hole application. In frequency domain induction measurements a narrowband excitation with different frequencies can typically employed to get response of the surrounding earth formations from different spatial areas of the formations. A larger number of turns and correspondingly higher inductance can be typically used for lower frequency band to increase magnetic dipole moment for energizing deeper areas. In this case a dissipative power loss in the antenna coil limits the magnetic dipole moment of the antenna.
U.S. Pat. No. 5,955,884 issued to Payton et al. describes a transient type of electromagnetic well logging instrument. The instrument has a transmitter antenna coupled to a source of current having a waveform adapted to induce transient electromagnetic induction effect in the earth formation surrounding the wellbore. When the transmitter current in transient induction instrument is switched abruptly, the signals appearing at a receiver is caused by the induction currents in the formation by, the currents caused the transmitted signal. A remarkable advantage of transient electromagnetic instrument is an ability to separate in time the response of different spatial areas. In order to facilitate measurement of the formation response from nearby to deep areas the transmitted signal should be substantially broadband. In this case the transmitter antenna should meet the requirements of a large dipole moment and an ability to be rapidly switched.
An attempt to implement induction measurement through casing of cased wells is presented in U.S. Pat. No. 5,426,367 issued to Martin et al. Generating magnetic flux in the surrounding earth formations is achieved by saturating a relatively small part of the casing thus making “magnetic window” to the magnetic flux generated by coil inside casing. The method is not suitable to generating strong magnetic dipoles to facilitate deep investigation.
Thus known in the art instruments do not give any satisfactory solution that would facilitate broad spatial range of excitations in the transient induction method.
Therefore it is one objective of the present invention to provide a solution for this problem. It is another objective is to make it work in presence of conductive support or conductive collar in case of MWD application. Yet another objective of the present invention is to provide a solution for the problem of induction transient logging of cased wells.